Method and equipment for purifying an extraction solution from aqueous entrainment and impurities

ABSTRACT

The invention relates to a method and equipment with which an organic solution extraction solution is purified from entrainment of aqueous solution and impurities during hydrometallurgical liquid-liquid extraction. The method treats an organic extraction solution, which is loaded with a valuable metal or valuable substance from the aqueous solution. The purpose is to carry out the physical separation of water droplets and the chemical removal of impurities from the organic extraction solution simultaneously.

FIELD OF THE INVENTION

The invention relates to a method and equipment with which an organicextraction solution is purified from entrainment of aqueous solution andimpurities during hydrometallurgical liquid-liquid extraction.

BACKGROUND OF THE INVENTION

During liquid-liquid extraction, an organic reagent solution is mixed,in an extraction cell (mixer-settler) or in a column, into an aqueoussolution, which contains a substance to be purified and concentrated assoluble, metal usually in ion form or as a complex together with severalimpurities. The valuable metal or substance to be refined reactsselectively with an organic extraction chemical, whereby it is separatedfrom the aqueous solution into the extraction chemical as a pure form.The valuable metal or substance can then be separated from the organicsolution back into the aqueous solution (stripping) with the invertedchemical reaction to extraction, from which aqueous solution it can berecovered again as a product e.g. by precipitation or reduction intometal.

The extraction process is thus mixing liquids physically insoluble ineach other into droplets or a dispersion in the mixing section of theextraction equipment, and after chemical mass transfer occurs thedroplets in the dispersion are made to coalesce i.e. to recombine backinto the original layers of liquid in the settling section or settler.Intensive mixing or a significant change in the surface chemistryconditions of the process may result in very small droplets, whichrequire a lot of time to disengage to its own liquid phase. Thesedroplets do not necessarily have time to disengage in the actualsettling section of the extraction step, but move further on in theprocess with the other phase. The inclusion of the original feedsolution (aqueous solution) in the organic solution as it enters laterprocess stages may weaken the purity of the final product and demandextra purification measures. Likewise the organic extractant may end uplost with the treated aqueous solution. In both cases the costefficiency of the process is lessened.

In particular, a tank has been used for the removal of aqueousentrainment from a organic solution after the extraction cells, in whichthe entrained water droplets fall towards the bottom of the tank due tothe force of gravity and the purified surface layer can be routed to thenext process stage, which tank is then called an after-settler. The tankcan also function simultaneously as a surge tank, which is needed toeven out changes in the volume of organic solution occurring in variousparts of the process. In this case the surface level of the solution inthe tank varies.

The actual purification method of the organic solution, scrubbing,occurs using mixer-settler cells, in which basically the chemicallybound impurities are removed by treating the organic solution withsuitable aqueous solutions. In this case therefore a dispersion ofextraction solution and aqueous solution is formed in order to achieve alarge liquid-liquid surface area, as in an extraction cell. Besideschemical scrubbing, water droplets are also removed or the impuritiescontained in them are diluted. A mixer-settler cell built for scrubbingpurposes generally consists of a pump, a mixer and a settling tank withits retaining fences, and is usually the size of an extraction cell.Changes in organic solution volume cannot be balanced with a scrubbingcell, so a separate surge tank as mentioned above is needed, which hasthe required volume capacity.

OBJECT OF THE INVENTION

The method according to the present invention treats an organicextraction solution from hydrometallurgical liquid-liquid extraction,which is loaded with a valuable metal or valuable substance from anaqueous solution. The purpose is to carry out the physical separation ofwater droplets and the chemical removal of impurities from the organicextraction solution simultaneously.

The organic solution to be purified is scrubbed with an acidic aqueoussolution. The aqueous solution may be fed into the organic solution evenbefore the solution is sent to the settling tank and/or it can be fedinto the organic solution at the front end of the tank. The organicsolution is made to discharge into the feed end in several separatesub-streams evenly across the whole width of the tank. In order toseparate the small water droplets from the extraction solution and towash it from impurities, the direction of the flow proceedinghorizontally towards the rear end of the tank is diverted obliquely fromtime to time to the vertical. Simultaneously the cross-sectional area ofthe flow is momentarily decreased several times while the direction ofthe separated solutions is deflected sideways by means of picket fences.The pure organic extraction solution and the aqueous solution areremoved from the settling tank from the rear end in several separatesub-streams.

When the organic solution and the scrubbing solution used for thispurification are made to proceed from the feed end of the settling tanktowards the rear end and when the solutions are made to proceed in avertical direction in addition to a horizontal one, the solutions becomewell mixed. At the same time as the direction of flow is made partiallyvertical, the cross-sectional area of the flow is momentarily reduced,whereby the small water droplets are made to combine into larger dropsand the scrubbing effect of the scrubbing solution is intensified. Thesolution stream also moves laterally when it moves through overlappingslots in the picket fence.

The settling equipment according to the invention consists of anessentially rectangular settling tank, which comprises a feed end and arear end, two sides and a bottom. At least one feed pipe, which isconnected at one end to the organic solution feed connection, isarranged into the settling tank feed end. The feed pipe is equipped withseveral separate discharge elements evenly spaced across the whole widthof the tank, after which several picket fences are located when viewedin the direction of flow. The picket fences are arranged so as to beinclined towards the rear end of the tank and each of them is made up ofseveral slotted elements extending from one side of the tank to theother. The width of the slots in each slotted element and their locationin relation to each other in the picket fence is arranged to alter inorder to change the direction of flow, at times diagonally verticaland/or sideways. The rear end of the tank is equipped with at least oneorganic solution outlet pipe, of which one end is connected to thecorresponding outlet connection. The outlet pipe in turn is equippedwith several suction elements to remove the scrubbed organic solutionevenly throughout the whole width of the tank. There is a well in thebottom of the rear of the tank to collect the aqueous solution. The tankwell is equipped with at least one aqueous solution outlet pipe, whichis again equipped with several suction elements to remove the aqueoussolution evenly throughout the whole width of the tank.

With the equipment according to the invention, the functions, whichordinarily have required both a separate after-settler and scrubbingequipment can now be performed in a single unit. One of the beneficialfeatures of the equipment is its function as organic solution volumeequalising tank for one extraction process unit. The tank also functionsas a safety tank, in which organic solution can be stored in emergencysituations, such as when there is a threat of fire or duringmalfunctions. The method and equipment of the invention are intended forapplication mainly in extraction processes operating horizontally, asdistinct from columns.

The essential features of the invention will be made apparent in theattached claims.

SUMMARY OF THE INVENTION

The equipment consists of a rectangular-based settling tank, whereorganic solution is fed into the feed end and scrubbed solution isdischarged from the other end. The height of the tank is such that itboth allows for the total volume during process operation and thus alarge variation in residence time, as well as acting as a storage tankfor all the organic solution in the process. The residence time of theextraction solution in the tank is around 15-30 min.

The infeed of the solution into the settling tank occurs from at leastone feed connection into at least one feed pipe, the “bypass manifold”.The settling tank is preferably located in the arrangement at a lowerlevel than the extraction stages, so that the solution feed takes placeadvantageously by free flow. Pumping is undesirable at this stage,because it makes the water seepage droplets in the extraction solutiondecrease in size even more than before. The feed pipe is equipped withseveral discharge elements so that the volume flow of the solutionentering the tank is divided evenly into several sub-streams. Thisavoids lateral flows and eddies that would disturb the free settling ofthe droplets. The discharge element may be either a pipe attached to thefeed pipe or an opening in the feed pipe. The organic solution is fedbelow the surface of the liquid directing it diagonally downwards in thefeed end to the aqueous layer at the bottom, whereby water coalescenceoccurs and a water contact surface is formed, to which the small waterdrops to be removed can join. If required, the bottom of the tank at thefeed end can be equipped with a well. At least part of the aqueoussolution for scrubbing the organic solution is preferably fed into theorganic solution before it is sent to the settling tank.

In order for the extraction solution and the scrubbing solution to bedispersed in each other, the flow rate of the extraction solutionsub-stream is 0.7-1.5 m/s, preferably 0.9-1.2 m/s. The feed pipes areplaced upwards from the bottom of the tank so that there is a clearancebelow them of between 1/20- 1/10 of the depth of the tank. Thedownward-directed flow circulates first towards the feed end, turningfrom there towards the rear of the tank.

The first flow-balancing picket fence is located across the tank afterthe feed pipe, and consists of vertical slotted elements, which overlapeach other. In the first two slotted elements of the first picket fencethe slotted zone is only in part of the elements and otherwise they aresolid. The third element consists of a full-height slotted zone. Thefence evens out the solution streams in the vertical and horizontaldirections so that the solution flow starts to advance as evenly aspossible, approaching plug flow in form.

The function of the first picket fence is to even out the solutionstreams in the vertical and horizontal directions so that the solutionflow starts to proceed evenly towards the rear of the tank. In addition,its function is to promote the separation of small water droplets orwater seepage from the organic solution. One purpose is also to improvethe contact between the extraction solution and the aqueous solutionthat will scrub it. Therefore this picket fence may also be called acontact fence.

In addition to the solution flow-equalising contact fence, at least one,preferably 3-5 additional picket fences are placed downstream, whichhave the function of both calming and directing the stream and alsoacting as impact surfaces, where the water droplets can combine intobigger droplets as they move through the slots in the fences. Theslotted elements in the picket fences are mainly the same type as thethird element of the first picket fence, or at least like the first,third and then every subsequent element of the fence. All the fences areinclined downstream, so that they direct the solution streams so thatthe water at the bottom of the tank rises along the inclined fence tointensify the scrubbing effect. In addition, there is a free area at therear of the tank, where no fences or other barriers are placed in orderto obtain as calm and non-turbulent a flow pattern as possible thusenabling the last droplets to settle before the solution dischargingpoint.

The picket fences consist of narrow upright slotted elements made oflamella plates placed vertically, which are described in detail later.In one fence 3-6 elements are placed consecutively, so that the slotsoverlap in the direction of flow and the solution also has to flowlaterally to contact as much of the element surface as possible.

In one embodiment of the present invention the second element of thepicket fences following the contact fence differs from the others inthat it is solid from the bottom of the tank upwards for a distance of20-10% of the height of the tank. In this way the solid part of theelement in the nearest picket fence to the contact fence is larger thanin the fences further away in the direction of flow. The slots in theslotted zone of the second element are also 30-10% wider than the otherelement slots in the same picket fence so that the widest slots are inthe picket fence elements following the contact fence.

The sloping of the picket fences together with the damming effect of thesecond fence element also in turn improves the contact between theextraction and scrubbing solution. The scrubbing solution has to risealong the lower section of the second element to the slotted zone, inwhich the flow occurring through the slots further disperses thescrubbing solution into the extraction solution. Thus part of the waterseepage entrained in the organic solution is made to impact continuouslywith the scrubbing solution and separate into it. The chemical effect ofthe extraction solution continues at the same time.

In one embodiment of the invention a baffle element placedperpendicularly upwards is arranged between the picket fences, where theheight of the element from the bottom upwards is 25-6% of the totalheight of the tank. The baffle element is higher the nearer it is to thecontact fence. Depending on the location of the baffle element it may besolid at the lower section and have a slotted zone in the top section orthe baffle element may be slotted throughout its height. The combinedeffect of the contact fence and the baffle element is that the wholestream of organic solution is forced to flow through the narrow slots ofthe contact fence or the baffle element at some stage, which intensifiesthe scrubbing effect of the solution.

In one embodiment of the invention the damming section of the secondelement of the picket fence is formed so that it is always larger thanthe damming effect of the baffle element before the picket fenceconcerned. The function of the baffle element, like that of the secondelement of the picket fence, is to dam up the aqueous solution(scrubbing solution) flowing into the bottom of the LO-cell, so that theorganic and aqueous solutions come into contact each other. This enablesthe mechanical cleaning of the organic solution from residual waterdrops and also chemical cleaning as a result of the acid contained inthe scrubbing solution.

In one embodiment of the invention mesh elements are placed in thepassage formed between the picket fence elements. The mesh elementpreferably extends from one side of the LO tank to the other, like thepicket fences. It is preferable to construct the mesh element fromseveral modules, which are replaceable. The mesh size of the meshelement is preferably in the region of 5-10 mm. The mesh element furtherintensifies the formation of large water droplets, which settle to thebottom of the LO tank.

The settling equipment has a well at the solution discharge end, inwhich a water layer separated from the organic solution and movingagainst its lower edge accumulates. The aqueous solution is partiallysent back to the front end of the tank, where it is fed again asdroplets into the incoming solution. The aqueous solution or scrubbingsolution is fed into the passages between the contact fence elements,preferably into the passage between the first and second elements. Asecond fraction of the water can be fed if necessary even before thisinto the scrubbing solution pipeline through suitable nozzles or freelyfrom above the surface. Thirdly, the impurity-rich aqueous solution isremoved from the process for instance by routing it to the extractionfeed solution (aqueous solution), so that the valuable metal orsubstance can be recovered.

The removal of small water droplets is therefore based in this method onseveral factors. Before the settling tank the water stream to be fedinto the pipeline is dispersed into the organic solution in droplets, ofa considerably larger size than the droplets to be removed. These dropstogether form a surface area onto which some of the small droplets cancoalesce. When the stream has proceeded to the feed end of the settlingtank, by directing the organic solution the aqueous layer at the bottomis made to disperse into drops again, which travel with the flow,settling towards the bottom while trapping other water droplets. Thewater drops moving in the solution to be purified also collide with thepicket fences and any mesh elements that may be between the elements,forming a continuous film of water on the surface: a hydrophilicsurface, which provides the water droplets with a convenient adhesivebase.

Extraction processes are used in conditions where the extractionchemicals work as selectively as possible, so that the desired valuablemetal or substance can be recovered in a sufficiently pure form. Mostly,however, various impure substances bind themselves chemically to theextractant in addition to the desired substance in such great quantitiesthat scrubbing methods have to be used to prevent the impurities fromproceeding up to the end product. In this way scrubbing solutions basedon the ion exchange effect, e.g. pH value, or those containing apurifying substance can be used, in order to displace impurities fromthe extraction chemicals.

In the equipment according to the invention, the above-mentionedchemical washing can be performed in combination with the physicalremoval of small water droplets. Aqueous solution containing valuablemetals or substances to be cleaned e.g. from elsewhere in the process,is added to the water for water droplet collection, so that the valuablemetal or substance is transferred in the ion exchange occurring duringtreatment to replace the impurities. Alternatively, the extractantcomplex containing impurities becomes unstable in the pH of the washwater, releasing the impurities into the scrubbing solution. The methodcan thus also be used to take care of the liquid balance of the processand to get the valuable metals or substances from the process watersback into circulation. The amount of aqueous solution circulating in thesettling tank is quite small in relation to the amount of extractionsolution, so the tank cannot be compared with the settler section inextraction. The amount of aqueous solution is around ⅙- 1/10 of theamount of organic solution when the scrubbing solution is fed into thetank, and even less if the purpose is to separate out only the waterdroplets in the organic solution. This equipment does not include amixing section typical of extraction steps either.

The scrubbed extraction solution is removed from the equipment bysuction with a pump through at least one outlet pipe, which is the sametype as the feed pipe. The solution is thus sucked up evenly across thewhole width of the tank via the suction elements connected to the outletpipe in several separate sub-streams, which ensures that the flowremains non-turbulent at the rear of the tank. The suction element maybe a pipe connected to the outlet pipe or an opening in the outlet pipe.Suction elements are preferably slanted upwards towards the rear of thetank, so that the suction direction slants downwards from the surface ofthe solution, but nevertheless below the surface. In the same way, theaqueous solution (scrubbing solution) that has separated to the bottomof the tank is removed via at least one outlet pipe and the watersuction elements connected to it in several separate sub-streams. Thesuction element may be a pipe connected to the water outlet pipe or anopening in the outlet pipe. The water suction elements are preferablyslanted towards the bottom i.e. the water suction streams occurdiagonally from the bottom upwards.

The chemical purification of organic solution used in liquid-liquidextraction processes in a buffer tank to equalize the solution circuitis not restricted to a metal extraction process of certain kind. Themethod and equipment described above are, however, highly suitable forexample when the valuable substance to be recovered is copper. The samekind of acidic wash is also suitable in most cases for the purificationof extraction solution loaded with metal. In sulphate-based processesthe oxidising acid used is sulphuric acid as one scrubbing solutioncomponent and the other component is generally the metal being extractedin the extraction process. When the final recovery of the metal inquestion occurs with the electrowinning principle, the electrolyte fromelectrolysis can be used to make the extraction process scrubbingsolution. When for instance the metal to be extracted is copper, theelectrolyte contains 30-60 g/l Cu and 150-200 g/l sulphuric acid.Electrolyte is added to pure water so that the H₂SO₄ content of thescrubbing solution to be fed into the settler is in the region of 20-50g/l.

The settling equipment according to the present invention i.e. anextraction solution scrubber tank with fittings, which for the sake ofsimplicity is referred to hereafter by the abbreviation LO tank (Organicsolution Scrubber Tank), is preferably utilised in an extraction processwhere the solution streams are large. The extractants used in therecovery of copper extract very little other metals besides copper, sothat an extraction solution is obtained that is almost pure enough withregard to copper. The meticulous removal of residual water dropscombined with a certain chemical scrub often raises the purity of theextractants used sufficiently for the subsequent process, i.e.electrolysis, nor is a separate scrubbing stage always necessary.

If however, the extraction solution contains a larger amount of harmfulsubstances, it should be treated further in a separatemixer-settler-type scrubbing step. In copper extraction these harmfulsubstances are iron, molybdenum and manganese. When the amount ofimpurities is such that in an ordinary configuration one scrubbing stepis not enough, it is now advantageous to use settling apparatusaccording to this invention in addition to a single scrubbing stage inorder to achieve sufficient purity in the extraction solution. In thisway the use of several scrubbing stages can be avoided. In somesituations sufficient scrubbing can only be achieved with a largequantity of scrubbing solution, which consumes water and increases thecirculation of metal via scrubbing. For instance many large copperextraction facilities are located in dry deserts where purified water isitself a significant cost factor. In addition, costs arise from coppercirculation when the rinse water used is either routed back to theextraction stage or to the leaching preceding it. In these kinds ofsituation the use of an LO tank improves the economy of the process.

DESCRIPTION OF DRAWINGS

The equipment of the invention is further described by means of theattached drawings, in which

FIG. 1 shows one arrangement of an extraction facility according to theinvention seen from above,

FIG. 2 shows a LO tank of the invention as a longitudinal cross-section,

FIG. 3 is a LO tank according to FIG. 2 seen from above, and

FIG. 4 shows another embodiment of the LO tank as a longitudinalcross-section.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows how the LO tank 1 i.e. the settling and scrubbing tank ofthe organic solution, is connected to the rest of the extractionprocess. The extraction process in the drawing consists of extractionsteps E1, E2 and E3, a LO tank, one scrubbing stage W and a strippingstage S. Each extraction, scrubbing and stripping stage includes eitherone or more mixers 2 and a settler 3 and the necessary pumps and piping.As the drawing shows, there is no mixer section in the LO tank, instead,the organic solution containing a valuable substance is brought thereand fed into the tank using a number of feed units 4 and outlet units 5that is sufficient for the amount of feed. As stated above, the actualscrubbing stage can be omitted if the amount of impurities in theorganic solution is small.

FIG. 2 shows an embodiment of the LO tank 1 of the invention in moredetail. The feed end 6 and rear end 7, bottom 8 and upper edge 9 of thetank are shown. In the bottom of the tank 1, there is an additional well10 at the rear end for the separated aqueous layer. The depth of thewell at the rear is around ⅙-⅓ of the depth of the rest of the tank. Theorganic solution solution is fed into one or more feed pipes 11 situatedin the feed end of the tank via feed unit(s) 4, the number of whichdepends on the amount of organic solution. In the drawing there are twofeed pipes. Each feed pipe is equipped with several discharge elements,which in this case are discharge pipes 12. The discharge pipes arepreferably directed diagonally downwards. The tank is equipped with atleast two picket fences, of which the first, the contact fence 13,differs somewhat in structure from the other picket fences 14. All thepicket fences are preferably inclined towards the rear of the tank. Thepreferred angle of inclination is about 45-70° to the horizontal.

The scrubbed organic solution in the rear 7 of the tank is recovered viaone or more organic solution outlet pipes 15, which are in turnconnected to corresponding outlet units. The scrubbed organic solutionis sucked evenly across the entire cross-section into the outlet pipesby means of suction pipes 16. The outlet pipes and their suction pipesare arranged the same way as the feed pipes and discharge pipes i.e. acertain amount of the solution to be removed is sucked out via eachoutlet pipe. The outlet pipes are located at the same point as the well10 at the bottom of the tank, but inside the organic solution. Thesuction pipes 16 are preferably directed diagonally upwards towards therear end 7. In the description of the invention the terms dischargepipes and suction pipes are used, but in principle these could also beopenings in the feed and outlet pipes.

In one application of the invention, a protective structure 17 seen inthe drawing is arranged on top of the outlet pipes, which consists of anessentially horizontal plate 18 on top of the outlet pipes and avertical plate 19 attached to its front edge. The vertical plate islocated in front of the first outlet pipe in the direction of flow andextends to about halfway down the pipe. The vertical plate may beperpendicular to the horizontal plate 18 as in the drawing or the jointmay be profiled as a curve. The horizontally-positioned plate extends alittle nearer to the rear end than the rearmost outlet pipe. Theprotective structure arranged on top of the outlet pipes ensures thatonly scrubbed organic solution flowing in the upper section that hascirculated near the rear of the LO tank is sucked out of the tank andinto the following stage.

The aqueous solution that has accumulated in the well 10 is also removedvia one or more aqueous outlet pipes 23 and corresponding aqueous outletunits and routed to one or more points in the process, as explainedabove.

The number of LO tank feed and outlet connections is determinedaccording to the amount of solution fed into the tank. FIG. 3 shows theLO tank as seen from above, where sides 21 and 22 are also seen. Theextraction solution is fed into the feed end of the tank, in this casevia two units 4 in the side 21 and removed via two outlet units 5 in therear end. Each feed unit 4 is in turn connected to a feed pipe or“bypass manifold” 11 in order to distribute the incoming organicsolution stream evenly over the entire width of the tank. If there areseveral feed pipes, the discharge pipes of each feed pipe feeds theorganic solution into its own sub-section. The number of sub-sections isthe same as the number of feed pipes. When the LO tank is wide, an evenfeed across the whole width of the tank without major pressurevariations is ensured by the use of several feed pipes and dischargepipes situated in their own sub-sections. According to FIG. 3 the firstfeed pipe extends only about halfway across the width of the LO tank andits discharge pipes feed the solution for about half the width of thetank. The second feed pipe extends as far as the opposite side 22 of thetank, but the organic solution discharge pipes 12 are located only onthe side of the tank where the first feed pipe does not reach.

The feed pipe or pipes are preferably placed so that they do not exactlytouch the feed end 6 of the LO tank, but come a little short of it. Thedischarge pipes 12 are correspondingly preferably directed obliquelydownwards towards the feed end. As a result, a solution circulation flowforms around the feed pipe. The length of the discharge pipe ispreferably at least twice the diameter of the pipe, so that thedischarge jets can be angled diagonally downwards towards the aqueouslayer forming on the bottom.

Correspondingly, the scrubbed organic solution in the rear end of thetank 7 is sucked evenly across the whole cross-section via one or twooutlet pipes 15, which are equipped with suction pipes 16. For reasonsof clarity the protective structure 17 has been omitted in the drawing.The outlet pipes and their suction pipes are arranged in the same way asthe feed pipes and discharge pipes i.e. as many parts of the solution tobe removed are sucked up via each outlet pipe as is required by thenumber of outlet pipes.

The aqueous solution that has accumulated in the well 10 is removed inexactly the same way via one or more aqueous outlet pipes 23, which arealso equipped with their own suction pipes 24. The aqueous suction pipesare preferably directed obliquely downwards. The suction pipes may alsobe directed to the rear section of the tank. The aqueous outlet pipesand their suction pipes are also arranged in the same way as the feedpipes and discharge pipes i.e. a certain amount of the solution to beremoved is sucked up via each outlet pipe. It is advantageous to removemore solution via the aqueous suction line than the amount that isseparated from or fed to the extraction solution, since in this way thepurity of the organic solution is ensured as regards aqueousentrainment. Thus some organic solution from the bottom of the organiclayer is also sucked up along with the aqueous solution. The amount oforganic solution sucked up with the aqueous solution is at the mostabout half the amount of aqueous solution sucked up. Some of the aqueoussolution, which consists mainly of scrubbing solution used for scrubbingthe organic solution, is preferably to recirculate into the organicsolution fed into the tank even before the latter is fed into the tank.Some of the scrubbing solution can be fed directly into the tank at thecontact fence. It is however also appropriate to remove a part of theaccumulated aqueous solution completely from the circuit from time totime, because it contains impurities that have dissolved out of theorganic solution, such as iron.

If the number of feed or outlet pipes is increased, the discharge andsuction pipes are distributed as described above. If there are threepipes, one third of the solution is fed from each pipe etc.

As shown in FIGS. 2 and 3 also, the LO tank is equipped with severalpicket fences 13, 14, which are set diagonally towards the rear of thetank. The purpose of these structures is to improve both the separationof the water seepage from the organic solution into larger droplets andto improve the contact between the extraction solution and the scrubbingsolution. Each picket fence consists of several elements in the samedirection.

The first picket fence 13 is located quite close to the organic solutionfeed pipes 11. It consists of at least three elements, extending fromone side of the LO tank to the other. FIG. 2 shows that the firstelement 25 of the contact fence is situated so as to extend as far asthe bottom 8 of the LO tank and that its upper edge reaches a heightwhich is preferably 50-70% of the height of the whole tank. About onethird of the upper section of the first element is provided with aslotted zone otherwise the element is solid. Vertical slots are arrangedin the slotted zone, with a preferred width of around 2-3 mm and adistance from each other that is 30-60 times the width of the slot. Onlya small amount of the organic solution flows through the slots, as therest flows above the element into the passage formed by the latter andthe following element. The second element 26 is situated at a depth sothat the distance between its lower edge and the bottom of the tank is15-20% of the height of the tank and the distance of the upper edge fromthe upper edge of the tank is around 12-17% of the height of the tank.About one third of the lower section of the second element is preferablyprovided with the same kind of vertical slotted zone as the uppersection of the first element, otherwise the element is solid. The narrowslots of the elements promote the formation of larger droplets from thewater seepage. The third element 27 of the contact fence is situated soas to extend to the bottom and its upper edge to about the same heightas the second element. The third element has vertical slots along thewhole height of the element, but their width is 40-60 mm and thedistance from each other is about twice that of the slot width. Thedistance of the passages left between the elements is basically thesame.

When scrubbing solution is fed directly into the LO tank, it is donepreferably by disseminating the droplets of scrubbing solution into theorganic solution at the point of the contact fence. The contact of thesolutions is further improved by guiding the scrubbing solution into thepassage 28 between the first and second elements.

It is further preferable to place 2-5 other picket fences 14 in the LOtank, to promote the growth of small droplets of aqueous solution andthe scrubbing of impurities from the organic solution. The subsequentpicket fences of the contact fence are largely similar to each otheri.e. they consist of several elements in the same direction andextending from one side of the tank to the other. The height of theelements is about the same as that of the third element 27 of thecontact fence, in other words they extend from the bottom of the tankupwards and the distance of their upper edge from the upper edge of thetank is around 12-17% of the height of the tank. The elements areprovided with the same type of slots as the third element of the contactfence, but the element slots are situated in relation to each other sothat they overlap, so that the distance the solution flows between theelements is as long as possible. The number of elements in each picketfence is 3-6.

FIG. 4 presents an embodiment of the invention where at least one baffleelement 29 placed perpendicularly upwards is arranged between the picketfences, with a height from the bottom 8 of 25-6% of the total height ofthe tank. Baffle elements are always placed between the picket fencesand the height is bigger, the nearer they are to the contact fence. Thusthe highest element is between the contact fence and the followingpicket fence and the second highest in the following gap. Depending onits location the baffle element may be solid in the lower section andhave a slotted zone in the top or it may have a slotted zone throughoutits height. The width of the slots in the slotted zone and the distancefrom each other is about the same as in the first and second elements ofthe contact fence. The width of the slots is thus 2-3 mm and thedistance between them 30-60 times the width of the slot. At maximum thesolid section is in the baffle element nearest the contact fence and itis around 40-60%. The proportion of solid section decreases in thedirection of flow of the tank and the slotted zone of the final baffleelement extends along the whole height of the element.

The invention is not restricted to the above-mentioned embodiments andit is possible to make adaptations and combinations of the aboveaccording to the patent claims within the scope and spirit of theinvention.

1. A method for purifying an organic extraction solution, comprising:providing a settling tank having a feed end and a discharge end anddefining a generally horizontal direction of flow from the feed end tothe discharge end, supplying an organic extraction solution containingdrops of aqueous solution and a valuable metal or substance separatedinto it during hydrometallurgical liquid-liquid extraction, for removingdrops of aqueous solution and other impurities from the organicextraction solution, to the settling tank at the feed end of thesettling tank in several separate sub-streams distributed evenly acrossthe whole width of the tank at the feed end thereof, whereby the organicextraction solution flows in a generally horizontal direction towardsthe discharge end of the settling tank, at several locations spacedapart along the generally horizontal direction of flow, diverting thedirection of flow of the organic extraction solution flowing generallyhorizontally from the feed end to the discharge end obliquely to thevertical, feeding scrubbing solution into the settling tank near or at alocation at which the direction of flow of the organic extractionsolution is diverted obliquely to the vertical, at several locationsspaced apart along the generally horizontal direction of flow,momentarily decreasing the cross-sectional area of the flow, deflectingthe direction of flow laterally by means of picket fences, removing thepure organic extraction solution from the rear end of the settling tankin several separate sub-streams, and removing the pure aqueous solutionfrom the rear end of the settling tank in several separate sub-streams,and wherein the amount of aqueous solution in the settling tank is lessthan about ⅙ of the amount of organic extraction solution.
 2. A methodaccording to claim 1, comprising feeding the organic extraction solutioninto the settling tank below the surface of the liquid and directingseparate sub-flows of the organic extraction solution diagonallydownwards towards the feed end of the tank for generating coalescence ofthe aqueous solution on the bottom of the tank and forming of a watercontact surface, in order to adhere small water droplets in the organicsolution.
 3. A method according to claim 1, comprising using an aqueoussolution containing acid as a scrubbing solution, where the amount ofacid is in the region of 20-50 g/l.
 4. A method according to claim 3,wherein the acid is sulphuric acid.
 5. A method according to claim 1,wherein the valuable substance contained in the extraction solution iscopper.
 6. A method according to claim 1, wherein the amount of aqueoussolution in the settling tank is in the region of ⅙- 1/10 of the amountof organic extraction solution.
 7. A method according to claim 1,wherein the scrubbed extraction solution is removed from the settlingtank non-turbulently by sucking it diagonally downwards in severalsub-streams from below the surface of liquid at the rear of the tank. 8.A method according to claim 1, wherein the aqueous solution is removedfrom the settling tank by sucking it diagonally upwards in severalsub-streams from the bottom of the rear of the tank.
 9. A methodaccording to claim 1, comprising feeding an acidic aqueous scrubbingsolution into the settling tank at a location at which the direction offlow of the organic extraction solution is diverted obliquely to thevertical.
 10. A method according to claim 9, wherein the acidic aqueousscrubbing solution contains in the region of 20-50 g/l acid.
 11. Amethod according to claim 10, wherein the acid is sulphuric acid.
 12. Amethod according to claim 1, comprising diverting the flow of organicextraction solution obliquely to the vertical by a picket fence locatedin the settling tank substantially closer to the feed end than to thedischarge end.
 13. A method according to claim 1, comprising scrubbingthe organic extraction solution with an acidic aqueous scrubbingsolution prior to routing the organic extraction solution into thesettling tank.
 14. Settling equipment for use in hydrometallurgicalliquid-liquid extraction for scrubbing an organic extraction solutionthat contains drops of aqueous solution and a valuable metal orsubstance separated into it during extraction, and where said equipmentcomprises: a substantially rectangular settling tank having a feed end,a rear end, two opposite sides and a bottom and defining a generallyhorizontal direction of flow from the feed end to the rear end, the rearend of the settling tank having a well in the bottom of the tank forcollection of aqueous solution, at least one organic solution feed unit,at least one outlet unit, at least one feed pipe at the feed end of thesettling tank, the feed pipe being connected at one end to the organicsolution feed unit and the feed pipe being equipped with severalseparate discharge elements evenly spaced across the entire width of thetank, several picket fences positioned in the settling tank and spacedapart in the generally horizontal direction of flow and inclined upwardstowards the rear end of the tank, wherein each picket fence extends fromone side of the tank to the other side of the tank and comprises severalslotted elements, whereby at each picket fence the direction of flow ofthe organic extraction solution flowing generally horizontally from thefeed end is diverted vertically and/or sideways, at least one organicsolution outlet pipe at the rear end of the settling tank, the outletpipe being connected at one end to the outlet unit and said outlet pipebeing equipped with several separate suction elements for removing thescrubbed organic solution evenly across the entire width of the tank, atleast one aqueous solution outlet pipe in the well at the rear end ofthe tank, the aqueous solution outlet pipe being equipped with separateseveral suction elements for removing the aqueous solution evenly acrossthe entire width of the tank, and a scrubbing solution feed pipe locatedfor feeding scrubbing solution into the tank near or at the mostupstream picket fence.
 15. Equipment according to claim 14, wherein thedischarge elements of the feed pipe are directed diagonally downwardstowards the feed end of the tank.
 16. Equipment according to claim 14,comprising a plurality of feed pipes at the feed end of the settlingtank, each feed pipe being equipped with several discharge elementsevenly spaced across a discrete sub-section of the tank width, thenumber of sub-sections being equal to the number of feed pipes. 17.Equipment according to claim 14, wherein the most upstream of the picketfences comprises a contact fence, which is closer to the feed pipe thanthe other picket fences, and the contact fence comprises first, secondand third slotted elements extending from one side of the settling tankto the other side of the settling tank and spaced apart in thehorizontal direction of flow, the first slotted element is between thefeed end of the settling tank and the second slotted element, and thesecond slotted element is between the first and third slotted elements.18. Equipment according to claim 17, wherein the first element of thecontact fence extends to the bottom of the tank, has an upper edge at aheight of at least about 50% of the height of the tank, has an uppersection that is slotted, and is otherwise imperforate.
 19. Equipmentaccording to claim 17, wherein the second element of the contact fencehas a lower edge located above the bottom of the tank at a height whichis about 15-20% of the height of the tank, has an upper edge at a heightof about 83-88% of the height of the tank, has a lower section that isslotted, and has an upper section that is imperforate.
 20. Equipmentaccording to claim 17, wherein the slots in the first and secondelements of the contact fence are about 2-3 mm wide and are spaced by adistance that is 30-60 times the slot width.
 21. Equipment according toclaim 17, wherein the third element of the contact fence extends to thebottom of the tank, has an upper edge at a height of about 83-88% of theheight of the tank, and is formed with slots over substantially itsentire height.
 22. Equipment according to claim 17, wherein the slots inthe third slotted element of the contact fence are 40-60 mm in width andthe slots are spaced by a distance about twice the slot width. 23.Equipment according to claim 14, comprising at least first and secondpicket fences, and wherein the first fence is between the feed end andthe other picket fences, the first picket fence is a contact fence, andthe second picket fence comprises slotted elements which extend to thebottom of the tank and at least one of which has an upper edge at aheight of about 83-88% of the height of the tank.
 24. Equipmentaccording to claim 23, wherein the second picket fence comprises atleast three slotted elements, at least the first and third slottedelements each include a slotted zone that extends over substantially theentire height of the slotted elements, the slot width is 40-60 mm andthe slots are spaced from each other by about twice the slot width. 25.Equipment according to claim 24, wherein the second slotted element ofthe second picket fence has a lower imperforate region that extendsupward from the bottom of the tank to 10-20% of the height of the tank.26. Equipment according to claim 24, wherein the second slotted elementof the second picket fence has a slotted zone in which the slots are10-30% wider than the slots in the first and second elements of thesecond picket fence.
 27. Equipment according to claim 14, comprising atleast first and second picket fences, wherein the first fence is betweenthe feed end and the other picket fences, and the equipment furthercomprises at least one vertical baffle element between the first andsecond picket fences, the baffle element having a height that is atleast 25% of the height of tank.
 28. Equipment according to claim 14,comprising at least first and second picket fences, and wherein thefirst fence is between the feed end and the other picket fences, and theequipment further comprises a means for introducing scrubbing liquidinto the tank at a location close to the first fence.
 29. Equipmentaccording to claim 14, comprising a mesh element positioned between theelements of at least one picket fence and extending across the tank fromone side to the other side.
 30. Equipment according to claim 29, whereinthe mesh element has mesh openings that are 5-10 mm in size. 31.Equipment according to claim 14, comprising a plurality of organicsolution outlet pipes serving respective sub-sections of the tank widthand each provided with suction elements.
 32. Equipment according toclaim 14, comprising a plurality of aqueous solution outlet pipesserving respective sub-sections of the tank width.
 33. A method ofcarrying out liquid-liquid extraction to separate metal contained in anaqueous solution from impurities in the aqueous solution, said methodcomprising: mixing the aqueous solution with an organic extractionreagent such that metal in the aqueous solution reacts selectively withthe organic extraction reagent and is separated from the aqueoussolution, separating the mixture of aqueous solution and extractionreagent to provide an aqueous solution containing impurities and anextraction reagent loaded with metal and containing droplets of aqueoussolution, supplying the extraction reagent to a feed end of a settlingtank in several separate sub-streams, the settling tank also having adischarge end and defining a generally horizontal direction of flow fromthe feed end to the discharge, the sub-streams being distributed evenlyacross the entire width of the tank at the feed end thereof, whereby theextraction reagent flows in a generally horizontal direction towards thedischarge end of the settling tank, at several locations spaced apartalong the generally horizontal direction of flow, diverting thedirection of flow of the extraction reagent flowing generallyhorizontally from the feed end to the discharge end obliquely to thevertical, feeding an aqueous scrubbing solution into the settling tanknear or at a location at which the direction of flow of the extractionreagent is diverted obliquely to the vertical, at several locationsspaced apart along the generally horizontal direction of flow,momentarily decreasing the cross-sectional area of the flow, deflectingthe direction of flow laterally by means of picket fences, removingextraction reagent from the rear end of the settling tank in severalseparate sub-streams, and removing aqueous solution from the rear end ofthe settling tank in several separate sub-stream.